Its been a while, but theres definitely been a few threads about farming greens in the past. I'll try to remember some of the interesting things that were found.
First is that you can use wowhead to see how ilvl works for scaling greens. Take
https://www.wowhead.com/item=15946/mystics-sphere&bonus=4280 for example. You can hover over "scales with ilvl" and click on the various versions to see what ilvls it drops at. It won't show the proper required level, but its based on the ilvl. You'll see it only sort of follows a pattern for ilvl. 10, 14, 18, 23, 27, 32, 37, 42, 47, etc. Weapons and armor definitely scale differently. I think for greens there are other differences in scaling as well (I know when the item was added was considered and discussed, I think pre-cata or something was different?).
Second is that the zone scaling window seems to have an affect. This is the thread you want to find if you can. Basically items won't scale below a certain point depending on the zone floor. So say you got to a 30-60 zone on your 19 (or 23/24 farmer) - the greens won't scale down that far. I can't remember the exact details, but if you can find that thread it had a theory that would be good for farming.
Things that drop from a specific zone/mob scale directly to the level of the looter. So Talon of Vultros only drops from Vultros. Shadowfang only drops in SFK.
For world drop blues...
Blue weapons drop at ...I think ilvls ending in 0/5. So you would see an ilvl 25 item that has required level of (ilvl - 8) for level 17 required.
Blue armor drops at ilvls ending in 8/3 with a required level of (ilvl - 8). So you can get a level 20 cloth belt with ilvl 28. Or a level 15 cloth belt with ilvl 23.
